Thermally and photo-induced changes in the valence states of vapour-deposited As2S3 films: A comparative photoemission study of As2S3 and As4S4

During thermal annealing or light irradiation, the changes in the valence states of vapour-deposited As₂S₃ and As₄S₄ films were observed by UPS. The experimental results reveal that an as-deposited As₂S₃ film contains considerable numbers of As₄S₄ molecular units, which polymerize or cross link to form a As₂S₃ glassy network on annealing or irradiation.     

Raman spectra of As4S4 polymorphs: Structural implications for amorphous As2S3 films

Raman-scattering spectra of α-As₄S₄ and β-As₄S₄ have been determined at 300 and 10 K. Although similar in overall aspect, the spectral signatures of the two polymorphs are clearly distinct. We have made a careful comparison of these first-order crystal line spectra to the sharp features reported in the Raman spectra of freshly-deposited films of amorphous As₂S₃. Prior proposals for the presence of As₄S₄ molecules in the unannealed films are supported by these comparisons, but recent contentions that actual microcrystals of β-As₄S₄ are present in the as-deposited material are clearly contradicted by the absence of any of the lattice-phonon lines which are prominent in the crystals at frequencies below 70 cm^-1.     

Band to band radiative recombination in amorphous As2S3

Radiative recombination from band states is observed in amorphous As₂S₃. The spectral dependence of the radiation shows that recombination occurs before thermalization takes place.     

Photoemission study of the density of valence states of amorphous As2S3 and Sb2S3

The densities of valence states of amorphous As₂S₃ and Sb₂S₃ have been investigated by means of X-ray photoemission and ultraviolet photoemission spectroscopy. The spectra are interpreted on the basis of existing band structure calculations.     

Photo-synthesis of As3S2 crystal from AsAs2S2 mixture

Existence of As₃S₂ compound which is stable in a certain temperature range higher than room temperature has been confirmed in AsAs₂S₂ system. Photo-synthesis effect of As₃S₂ crystal from As and As₂S₂ mixture, namely light induced conversion from a crystalline state to another crystalline state, has also been investigated.     

Photoelectron spectra of As4S3, P4S3, P4Se3 and As4O6

The He(I) UV photoelectron spectra of As₄S₃, P₄S₃, P₄Se₃ and As₄O₆ are reported in this paper. The spectra of As₄S₃ and the isostructural molecules P₄S₃ and P₄Se₃ comprise seven broad but clearly defined peaks, whilst the spectrum of As₄O₆ consists of six peaks and two shoulders. By comparing the spectra with one another and with the photoelectron spectra of other arsenic and phosphorus compounds, the character and symmetry species of the highest-energy occupied molecular orbitals of the molecules have been assigned.     

Photoelastic properties of amorphous As2S3

The piezobirefringence of amorphous As₂S₃ was measured in the wavelength range 0.69 to 1.15 μ, and the pressure dependence of the index of refraction was determined at 0.633 and 1.15μ. The dispersion of the photoelastic constants of As₂S₃ is discussed using a one-oscillator model, and compared with the results of a similar analysis performed on GaAs.     

Conductivity of the chalcogenide semi-insulator system: As2S3Sb2S3

The conductivities of mixed As₂S₃Sb₂S₃ thin films are discussed. Temperature independent a.c. data are interpreted in terms of a hopping mechanism where correlation effects are important. Excepting As₂S₃, which exhibits extrinsic behaviour, the dielectric constant, a.c. conductivity and d.c. conductivity pre-exponential factor, all scale approximately linearly with composition.     

Photodarkening in glassy As2S3 under pressure

Photodarkening phenomena have been studied in glassy As₂S₃ films under pressure up to 3 kbar. A remarkable shift of the absorption edge to lower energy by the band gap illumination is observed under pressure. The shift is quenched even after releasing pressure. X-ray diffraction studies have also been performed at various pressures. The first sharp diffraction peak disappears at 50 kbar. The origin of the observed enhancement of the photodarkening may be associated with the increase in the interlayer correlation by application of pressure.     

Acousto-induced structural change in amorphous As2S3

The acoustic flux injection method was applied to amorphous As₂S₃. The velocity and the attenuation constant for shear wave at ～ 100MHz were measured through the observation of the Brillouin scattering of the light beam from a He-Ne laser. The optical transmission around absorption edge was found to change after the injection of acoustic fluxes. It was also found that the sound velocity decreased with increasing injection time of acoustic fluxes and the amount of the change of the sound velocity reached ～ 20% after 10⁵ injections. The changes in the optical transmission and the sound velocity tended to be erased by annealing below the glass transition temperature. These phenomena are considered to be due to structural changes caused by the injection of strong acoustic fluxes.     

Resonance Raman scattering from amorphous As2S3

Resonance Raman scattering from amorphous As₂S₃, obtained by temperature tuning the band edge through the He-Ne 6328 Å laser line, is reported.     

Electronic structure of As4S5 as a local structural model for amorphous As2S3 film

We have studied the electronic structure of As₄S₅ molecule as a model of the local pyramidal structure in amorphous As₂S₃ film. Emphasis has been put on the analysis of the behaviour of a trapped electron in this model concerning the consequent structural relaxation. It has been found that As₄S₅ species has affinity for an excess electron, and that the attachment of an excess electron promotes the cleavages of specific As-S bonds.     

Reversible photostructural change in melt-quenched As2S3 glass

Reversible phtodarkening and photostructural change accomplished by repeated cycles of band-gap illumination and annealing have been observed in melt-quenched As₂S₃ glass for the first time. The magnitudes of photo-induced shift of optical absorption edge as well as the fraction of photo-expansion observed in melt-quenched As₂S₃ glass coincide quantitatively with those observed in well-annealed evaporated As₂S₃ glass. The experimental data involving results of crystalline As₂S₃ demonstrate that the reversible photostructural change is “unique” to amorphous state and is not affected by the difference in disordered structure originated from preparation technique.     

Photoluminescence decay affected by variable range hopping in band tail in amorphous As2S3

Photoluminescence decay was observed with various excitation energies and at various temperatures in amorphous As₂S₃. The decay at high temperature or with the high excitation energy had the $\text{t}{}^{\mathrm{<mtext>-3</mtext><mtext>2</mtext>}}$ long time behavior. This decay is concluded to arise from the combination of the tunneling recombination and the variable range hopping of carriers in the band tail.     

Photo-lightening effect in crystalline As2S2 films

A photo-induced shift of the optical transmission edge to a shorter wave-length in crystalline As₂S₂ films has been studied at room temperature and liq. N₂ temperature. This effect has been investigated by X-ray diffraction and scanning electron micrograph observations.     

Hole mobility in evaporated layers of As2S3.CdI2

The hole drift mobility in As₂S₃.CdI₂ has been determined using Spear's method. The room-temperature value is 10^?4$\text{cm}{}^2\text{Vs}$, the activation energy 0.2 eV. The hole yield appears to be controlled by geminate recombination.     

Photo- and thermal-diffusions of metals into As2S3 glass

Photo- and thermal-diffusion effects of Ag and Cu into As₂S₃ bulk glass have been studied. It has been clarified that the photo-diffusion process is attributed to a purely optical effect. Some differences in diffusing process between Ag and Cu have been clearly shown both for the photo- and thermal-diffusions.     

Density of valence states of As2O3 and Sb2O3

The densities of valence states of As₂O₃ and Sb₂O₃ have been measured by means of X-ray photoemission, ultraviolet photoemission, and soft X-ray emission spectroscopy. The spectra can be interpreted on the basis of existing models for the As and Sb chalcogenides.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.     

Reversible photo-induced volume changes in evaporated As2S3 and As4Se5Ge1 films

Reversible photo-induced volume changes have been observed accompanying reversible optical absorption edge shift produced by successive cycles of band gap illumination and annealing for well-annealed evaporated As₂S₃ and As₄Se₅Ge₁ films. The As₂S₃ film shows an increase in its volume by illumination, while As₄Se₅Ge₁ film shows a decrease. Qualitative discussion has been given in connection with pressure-induced optical constant change.